If the Moon Were Replaced by Planets

What is If the Moon Were Replaced by Planets?

If the Moon Were Replaced by Planets is a planet size simulator built as a true-angular-size 3D night sky scene. It asks a very specific question: what would familiar solar system bodies look like if they occupied the Moon’s distance of 384,400 km from Earth? The app shows the Moon, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and the Sun in that same position so you can compare their apparent size against the real full Moon.

This is part of the Tiny to Vast interactive science tool collection and is made with Three.js and WebGL. The experience is visual and exploratory rather than numerical only, which makes the planet size simulator useful for teaching scale, angular diameter, and why distant objects often appear tiny even when they are physically enormous.

Key features

The planet size simulator focuses on one core idea: apparent size depends on both diameter and distance. The app keeps the viewing setup centered on the Moon’s distance, then swaps in other bodies so the sky changes in a way that feels immediate.

• True angular-size rendering at 384,400 km for direct sky comparison.
• Night landscape backdrop so the planet size simulator feels like an actual observing scene.
• Drag, arrow-key, scroll, and pinch controls for looking around the sky.
• Phase adjustment to light the object like a waxing or waning Moon.
• Distance range from 0.25× to 16× the Moon’s distance for extra scale exploration.
• A built-in size table that lists each body’s diameter and apparent sky width.
• A special Sun handling mode, since the Sun’s radius is larger than the Moon’s distance and Earth would be inside it at the default setup.

The simulator also explains the math behind the view. Apparent size is shown as the full angular diameter of the visible disk, using the sphere limb calculation 2·asin(radius ÷ distance). That makes the planet size simulator more than a visual toy; it also links the image to the geometry behind it.

How to use it

Using the planet size simulator is straightforward, and the interface is built around quick visual comparisons.

1. Start with the default Moon view to understand the baseline size.
2. Pick another body such as Jupiter, Saturn, or Mars and compare it in the same sky.
3. Drag, use the arrow keys, scroll, or pinch to change where you are looking.
4. Adjust the phase to match different lighting conditions.
5. Move the distance slider from 0.25× to 16× the Moon’s distance to see how quickly apparent size changes.
6. Read the size table if you want the numeric angular width for each body.

The most useful habit is to switch between bodies while keeping the viewpoint similar. That makes the planet size simulator a practical way to test intuition about how much sky a large object actually covers.

Who it is for

This planet size simulator is a good fit for people who learn visually and want a concrete answer to a scale question.

• Astronomy hobbyists who want to picture the solar system at human eye level.
• Teachers explaining angular size, distance, and apparent diameter.
• Students comparing the Moon with planets in a single visual frame.
• Science communicators looking for an easy demo about scale.
• Curious readers who have ever wondered whether Jupiter would look huge or still modest in the sky.

Because the app combines a night-sky view with specific measurements, the planet size simulator can support classroom discussion as well as casual exploration. It is especially helpful when a screenshot or a number alone is not enough to make the size difference feel real.

What to know before you use it

This planet size simulator is accurate for visual appearance, but it does not model every physical consequence of putting a planet at the Moon’s distance. The site explicitly notes that a gas giant there would create extreme tides and destabilize Earth. In other words, the app answers what your eyes would see, not what would be safe or survivable.

A few details matter before you rely on the scene:

• The Sun scenario is only visualized after moving it far enough away for its disk to fit in view, because at 384,400 km Earth would be inside the Sun.
• Saturn’s rings are represented as an angular-size case, and their face-on span is much wider than the planet itself.
• The textures come from the Solar System Scope texture pack, which is based on NASA imagery and elevation data under CC BY 4.0.
• The app is a simulation, so it is best for concept building, not for mission planning or scientific hazard analysis.
• If you need exact observational data for the real Moon, planets, or Sun, use an official ephemeris or astronomy reference instead of this planet size simulator.

For anyone trying to build intuition about planetary scale, the planet size simulator does an excellent job of turning abstract angles into something you can literally look around and compare. It is a compact, focused way to see how the Moon distance changes the sky, and it closes the loop between geometry and visual experience.